subshell is represented by a box and the electron is represented by an arrow ( ↑ ) a positive spin or an arrow ( ↓ ) a negative spin. The advantage of second notation over the first is that it represents all the four quantum numbers. The hydrogen atom has only one electron which goes in the orbital with the lowest energy, namely s . The electronic configuration of the hydrogen atom is s meaning that it has one electron in the s orbital.
The second electron in helium (He) can also occupy the s orbital. Its configuration is, therefore, s . As mentioned above, the two electrons differ from each other with opposite spin, as can be seen from the orbital diagram. The third electron of lithium (Li) is not allowed in the s orbital because of Pauli exclusion principle.
It, therefore, takes the next available choice, namely the s orbital. The electronic configuration of Li is s s . The s orbital can accommodate one more electron. The configuration of beryllium (Be) atom is, therefore, s s (see Table .
, page for the electronic configurations of elements). In the next six elements—boron (B, s s p ), carbon (C, s s p ), nitrogen (N, s s p ), oxygen (O, s s p ), fluorine (F, s s p ) and neon (Ne, s s p ), the p orbitals get progressively filled. This process is completed with the neon atom. The orbital picture of these elements can be represented as follows : The electronic configuration of the elements sodium (Na, s s p s ) to argon (Ar, s s p s p ),